1. Field of the Invention
The present invention relates to a demodulating method for multi-frequency quadrature modulation in digital wireless communications.
2. Description of the Background
A technique of multi-frequency quadrature modulation, which presents strong resistance to multipath distortion has been investigated in the fields of ground wave digital broadcasting, high-speed digital mobile wireless communications and subcarrier transmission. The technique of multi-frequency quadrature modulation is a method for transmission of digital data by frequency multiplexed transmission using a plurality of carrier wave frequencies which are distributed at intervals of the reciprocal of a symbol period.
FIG. 1 shows a configuration for a conventional method of multi-frequency quadrature modulation. The operation of the conventional method of multi-frequency quadrature modulation will be described hereinbelow with reference to FIG. 1.
A binary digital signal input from an input terminal 1 in a transmitter 10 is processed in a digital modulator 11 where the input signal is phase modulated (modulated by PSK: phase shift keying) or quadrature amplitude modulated (modulated by QAM: quadrature amplitude modulation) so as to be converted into an equivalent low-band symbol of the signal.
The modulated symbol is input to a serial-to-parallel converter (S/P) 12 where it is converted into N number of symbol streams which each have a transmission rate 1/N as much as that of the input symbol stream. These streams are processed in an inverse discrete Fourier transformer (IDFT) 13 where the symbol streams are imprinted onto subcarriers having corresponding frequencies and composited to be output.
The output signal is a summation signal of a plurality of modulated signals which are distributed at intervals of the reciprocal of the symbol period. The output signal from IDFT 13 is converted into serial data by a parallel-to-serial converter 14. Guard intervals are inserted into the converted data by a guard interval inserting section 15.
The signal with guard intervals inserted therein is then quadrature modulated by a quadrature modulator 16, and the resultant signal is output from a modulated signal output terminal 2 so as to be transmitted.
A receiver 20 operates in the reverse order to that performed in transmitter 10 so as to interpret the transmitted data stream. First, the received signal is input from an input terminal 3 and is quadrature demodulated by a quadrature demodulator 21. The quadrature demodulated signal is stripped of the guard interval components through a guard interval remover 22.
A symbol synchronous signal is generated by a symbol synchronous signal generating circuit 27. The signal which has been stripped of guard intervals is converted into parallel data by a serial-to-parallel converting circuit 23. The converted data is input to a discrete Fourier transformer (DFT) 24. In DFT circuit 24, the received signal is separated into equivalent low-band signals each corresponding to a subchannel so as to be output as parallel data consisting of N symbols.
These symbols are converted into the original serial data by a parallel-to-serial converter (P/S) 25, and then subjected to a judgment by a digital demodulator 26 of whether they are of a PSK signal or QAM signal. The result is output from the received data output terminal.
In the multi-frequency quadrature modulating method, the transmission rate for each subchannel is low enough so that the signal will be little affected by delayed multi-pass waves. Further, guard intervals are provided in order to completely eliminate intra-code interference due to delayed waves. FIG. 2 shows an overall waveform of a modulated signal in the multi-frequency quadrature modulating method.
As understood from FIG. 2, the guard interval has the same waveform interposed as that in the rear end of the observation interval signal. The provision of the guard interval prevents interference due to delayed waves which have been delayed by the length of the guard interval or less, making it possible to inhibit degradation of transmission characteristics.
On the other hand, the multi-frequency quadrature modulation signal is markedly degraded in its transmission characteristics due to frequency offset between transmitting and receiving equipment and due to time-dependent variation in amplitude and phase caused by the propagation path.
Concerning the frequency offset and relatively slow time-dependent variations in phase, various frequency synchronizing methods have been investigated and their validity has been proved. However, the conventional methods need a very long time to establish frequency synchronization, so that they are not effective for high speed variations in phase.
In a multi-pass phasing propagation path, if transmission beyond the coherence band width is performed, time-dependent variation in amplitude and phase of a signal occurring within its transmission band differs from others for each frequency band range. Therefore, it is impossible for the conventional frequency synchronizing methods to compensate them.
Consequently, when a multi-frequency quadrature modulation signal which has undergone high-speed and frequency selective phasing is received, time-dependent variation in amplitude and phase caused by the propagation path needs to be corrected for each frequency band and then demodulated